Multiple-input-multiple-output (MIMO) wireless systems exploiting multiple antennas for transmitting and/or receiving data have attracted increasing interest due to their potential for increased capacity in rich multipath environments. Such systems can be used to enable enhanced communication performance (i.e. improved signal quality and reliability) by use of multi-path propagation without requiring additional spectrum bandwidth. This has been a well-known and well-used solution to achieve high data rate communications employing either 2G or 3G communication standards.
For indoor wireless applications such as router devices, external dipole and monopole antennas are widely used with high-gain, omni-directional dipole arrays and collinear antennas being the most popular.
However, there are very few portable devices with MIMO systems on the market and this is mainly because of the complications around gathering several radiators in a small device (due to the small allocated space of the terminal), while maintaining the required isolation between each radiator.
One of the interesting solutions to this problem involves the use of balanced radiators which do not require a ground plane (and especially ground plane currents) to efficiently radiate. Over recent years, balanced antenna systems have attracted increasing interest to mobile phone antenna designers because of their stable performance when held adjacent to the human body. In this type of antenna, only balanced currents flow on the antenna element, thus remarkably reducing the effect of current flow on the phone chassis and the influence of the human body on antenna performance can be made small.
The structure of a balanced antenna system typically comprises a radiating element which is fed by a balanced line or “balun” (which is configured to convert a single unbalanced signal into two differential balanced signals or vice versa). Such balanced antennas have been successfully applied to design a two-element structure which operates at 2.45 GHz and 5.2 GHz in PDA and laptop devices. However, it is currently considered difficult (or perhaps even impossible) to implement such structures at lower frequencies; for instance in either DVB-H, GSM or UMTS mobile phones because of the physical size of the corresponding resonance.
There is therefore a need for a compact balanced antenna design for MIMO applications in mobile devices which can provide coverage across the whole spectrum of frequency ranges required by service providers and customers. Consequently, an aim of the present invention is to provide a balanced antenna system which helps to address the above-mentioned problems.